1. Field of the Invention
This invention relates to curable epoxy-based compositions such as those having an epoxy compound which has two or more epoxy groups per molecule, a polythiol compound which has two or more thiol groups per molecule, a latent hardener, and at least one solid organic acid which is substantially insoluble in a mixture of the foregoing components at room temperature. The solid organic acid may be selected from the group consisting of: aliphatic, cycloaliphatic and aromatic carboxylic acids and derivatives thereof, aliphatic, cycloaliphatic, and aromatic quinones and derivatives thereof, phenols and derivatives thereof and enolisable aliphatic, cycloaliphatic and aromatic compounds and derivatives thereof. The solid oraganic acid should have a pKa of less than or equal to about 12.0, suitably less than or equal to 10, such as less than or equal to about 9.0, and often less than or equal to about 7.5.
In another aspect of this invention, there are provided curable one-part epoxy-based compositions with improved rheological properties, such as improved shelf-life stability, particularly with respect to yield point maintenance over time, viscosity maintenance over time, and room temperature pot life.
2. Brief Description of Related Technology
Curable epoxy-based compositions are well known. Such compositions are used as adhesives, coating agents, sealing agents and may also be used as casting agents. Epoxy-based compositions are also used in the electronics industry for the fabrication of heat resistant printed circuit laminates for printed circuit boards (pcbs). One use of curable epoxy compositions is to bond surface mount components to pcbs.
Epoxy/polythiol-containing compositions conventionally have been used as two-part compositions. This was due at least in part to the instability of a one-part composition, having an epoxy resin and a polythiol component and a liquid (soluble) tertiary amine curing agent or hardener. One-part compositions of this sort where the epoxy resin-polythiol and the curing agent or hardener were admixed at room temperature had working or “pot” lives on the order of minutes to a few hours. These properties impose practical restrictions on end-use applications of such compositions. Accordingly, many traditional epoxy/polythiol compositions have been formulated as two-part compositions.
Commercially available latent curing agents used in one-part epoxy resin adhesive formulations ordinarily provide such formulations with a combination of good storage stability and moderate reactivity at elevated temperatures. Examples of such commercially available latent curing agents include dicyandiamide and dibasic acid dihydrazide. These curing agents are useful in formulating epoxy resin compositions with excellent storage stability. However to achieve cure, these curing agents ordinarily require heating to temperatures greater than 150° C. for extended periods of time.
U.S. Pat. No. 5,430,112 (Sakata) discloses epoxy resin/polythiol compositions which are reported to display enhanced stability, i.e. an extended pot life, if (a) a solid dispersion-type amine adduct latent curing accelerator, or (b) the product of a reaction between a compound which contains one or more isocyanate groups in its molecule and a compound which has two or more primary and/or secondary amino groups in its molecule, is used. The compounds (a) and (b) above are each reported to act as a “latent hardener”, being activatable at higher temperatures. In particular, the composition disclosed in the '112 patent contains (1) an epoxy resin which has two or more epoxy groups in its molecule, (2) a polythiol compound which has two or more thiol groups in its molecule and (3) an accelerator which is (a) a solid dispersion-type amine adduct latent curing accelerator, or (b) the product of a reaction between a compound which contains one or more isocyanate groups in its molecule and a compound which has at least one primary and/or secondary amino groups in its molecule. Examples given of commercially available solid dispersion-type amine adduct latent curing accelerators are those sold under the trade names Ajicure PN-H or Ajicure PN-23 (commercially available from Ajinomoto Co., Inc., Tokyo, Japan). The compositions containing these amine adduct latent curing accelerators show improved room temperature stability over conventional formulations based on liquid or soluble tertiary amine curing agents. However, in practice such compositions with a pot life of in excess of 1 week at room temperature, show a poor ability to cure, i.e. their ability to cure in less than 30 mins at 80° C. is poor.
The stability of an epoxy resin/polythiol composition of the '112 patent is reported to be improved by the use of a solid dispersion-type amine adduct latent curing accelerator and/or the product of a reaction between an isocyanate and an amine containing group, though compositions containing Ajicure PN-23 are not described. However, improved stability, for at least the commercially available Ajicure PN-H, is achieved at the expense of gel time, i.e. greater stability is achieved only with an undesirable effect of increase in gel time.
The '112 patent also describes the use of liquid or solid organic or inorganic acids for surface treating the latent hardener (the amine adduct) and for use in making the latent hardener. The treatment of the hardener with an acid is designed to neutralise active basic materials on the surface of the hardener particles as the hardener is ordinarily in a solid state. The organic or inorganic acid is often in a liquid state or in a solution to allow for the surface treatment, or for making the latent hardener.
An English-language abstract (produced by Derwent) (accession no. 86-229126) for Japanese laid-open patent application no. S61-159417 (Japanese Patent No. 92014701) discloses a two-part curable epoxy resin composition containing, epoxy compounds having on average above one epoxy group in one molecule, thiol compounds containing about one thiol group in one molecule as a hardener (though not a latent one), amines as a curing accelerator and mercapto-organic acids containing one carboxylic group and one thio group in one molecule as a curing retarder.
As regards increasing the gel times for two-part epoxy compositions Japanese patent publication no. 56057820 describes such a composition which contains an epoxy compound, a thiol compound as curing agent, an amine as curing accelerator, and an acid, added to retard the curing reaction. The composition is not suitable for formulation as a one-part composition as the amines used are not latent, and as such cause the composition to cure within a few minutes of the two parts of the composition being brought together. The JP '820 publication appears to be concerned with providing two-part compositions with increased gel times achieved when the two parts are brought together. This allows the mixed composition to be used for longer periods. The increased gel times are achieved by the addition of the acid component such as liquid acids and Lewis acids. The JP '820 publication does not appear to be concerned with providing storage stable compositions, and does not appear to teach achieving storage stability concomitantly with retention of relatively short gel times.
In the electronics industry, it is desirable to provide epoxy-based compositions with thermal cure profiles tailored to specific application temperature requirements. Such cure profile tailoring assists in maintaining the integrity of the electronic components during the bonding process of the components to a pcb. In addition, it is desirable for such compositions to have extended room temperature pot lives so that the composition may be repeatedly applied to the surface of the pcb. This prolongs the useable application life of the compositions, thereby ensuring reproducible dispensing properties.
Advances in the electronics industry have made precise deposition of surface mount adhesives a critical processing parameter, particularly in view of high throughput demand and process efficiency. The increasingly popular smaller-sized microelectronic components has made precise deposition of solder or adhesives onto circuit boards for chip attachment that much more important. Where precise adhesive deposition does not occur—either due to adhesive deposition technique imprecision, or spreading of the adhesive due to inappropriate rheological properties for the particular application, or both—surface mounting of components on pcbs may not occur at all, and even when mounting does occur, the mounting may not occur in a commercially-acceptable manner.
With certain applications, such as the applications in the electronics industry mentioned above, it has also become desirable for epoxy-based compositions to have a defined structural integrity. One way to achieve this is through the addition of a thixotropy-conferring agent, such as a clay or a silica, a large number of which are well-known. Indeed. Degussa makes available commercially a number of treated fumed silicas under the tradename “AEROSIL”, and has suggested their use to impart in epoxy resins a thickening and thixotropic effect. See also, C. D. Wright and J. M. Muggee, “Epoxy Structural Adhesives”, in Structural Adhesives: Chemistry and Technology, S. R. Hartshorn, ed., 113-79, 131 (1986).
Heretofore, the desirability of balancing reactivity with pot life in curable one-component epoxy-based compositions has been recognised. For instance, U.S. Pat. No. 3,597,410 (Lieske) discloses a method of prolonging the period of reaction of hardenable mixtures based on hardenable epoxide resins by providing an amount of a barbituric compound effective to retard cure. The compositions are otherwise relatively slow curing at elevated temperatures. The compositions of the '410 patent include hardenable epoxides containing more than one epoxide group in the molecule, together with an organic polycarboxylic acid anhydride epoxide resin hardener and an amount of a barbituric compound effective to retard cure. The object of the '410 patent is said to be prolonging the period of reaction or pot life, and retarding the hardening of the epoxy composition at elevated temperatures.
U.S. Pat. No. 5,130,407 (Lee) relates to an epoxy resin composition used for the fabrication of a heat resistant circuit laminate for printed circuit boards. The composition used is a modified epoxy resin which is obtained by reacting an epoxy resin with a mono-nucleus N-heterocyclic fatty compound as a chain extender, a polyfunctional epoxy resin and curing agents. Barbituric acid is mentioned as a possible chain extender.
U.S. Pat. No. 5,268,432 (Pan) relates to a heat resistant adhesive composition containing a bismaleimide in a composition which may be modified by barbituric acid for bonding a 3-layer flexible printed circuit.
United Kingdom Patent Application No. 2 287 940 refers to a liquid epoxy resin composition for bonding electronic parts onto printed wiring boards using a dispenser. The composition includes an epoxy resin, an amine curing agent, an inorganic filler (such as calcined talc) and an organic rheology additive (such as modified castor oil or an organic amide).
European Patent Application No. 459 614 refers to an epoxy resin composition of an epoxy resin, an amine-type latent curing agent and a hydrophilic silica. The compositions, having good shape retention, may be prepared by blending the components until a thixotropy index of 2 or less is achieved. The thixotropy index is given in the EP '614 application as a ratio of the viscosity measured at 25° C. and 0.5 rpm over the viscosity measured at 25° C. and 5 rpm. In addition, an index of the degree of viscosity change over time (“the thixotropy index changing ratio”) is given in the EP '614 application.
Notwithstanding the state of the art it would still be desirable to provide epoxy compositions with improved storage stability, such as improved room temperature pot life and with extended working lives and relatively short curing times. It would also be desirable to provide compositions with improved rheological properties, such as improved shelf-life stability, particularly with respect to yield point maintenance over time, viscosity maintenance over time, in addition to storage stability. Compositions having some or all of these properties are very useful commercially, as noted above.